1. Field of the Invention
The present invention relates to a magnetic recording medium having a structure that a magnetic layer made of a metal magnetic film is formed on a non-magnetic support member thereof to serve as a magnetic tape or a magnetic disk with which a short-wavelength magnetic recording signal can satisfactorily be recorded and reproduced (a dense recording can be performed) to serves as, for example, a high-band 8-mm video tape, a digital video tape or the like.
2. Description of the Related Art
A so-called ferromagnetic thin metal film type magnetic recording medium has a thin film made of alloy or an oxide of iron, cobalt, nickel or their combination and formed, on a non-magnetic support member thereof, by a vacuum film forming method, that is a PVD (Physical Vapor Deposition) method, such as a vacuum evaporation method, a sputtering method or an ion plating method. The ferromagnetic thin metal film type magnetic recording medium, containing no binder, enables the recording density to significantly be improved as compared with a so-called coating-type magnetic recording medium which is manufactured by coating a non-magnetic support member with a magnetic coating containing metal magnetic powder dispersed in a binder and by drying the magnetic coating. However, the above-mentioned improvement in the recording density (that is, raising the density) requires minimization of a spacing loss by reducing the magnetic gap of the magnetic recording head and by flattening and smoothing the surface of the magnetic recording medium.
If the surface of the non-magnetic support member is excessively flattened and smoothed in a case where the non-magnetic support member has a usual single layer structure, the sliding smoothness (the moving easiness) deteriorates and adhesion takes place. In this case, there arises a problem in that the workability (the handling easiness) deteriorates.
As a method which is capable of overcoming the above-mentioned problem, a method is known which has the steps of forming (applying) another layer on a surface of the single layer structure non-magnetic support member opposite to the surface on which the magnetic layer is formed and making the formed surface to be a relatively rough surface. However, the above-mentioned method suffers from the coating layer being easily cut, thus resulting in cut chips causing dropout to take place.
To overcome the above-mentioned problem, an attempt has been made to improve the sliding smoothness (the handling easiness) by employing a structure that the non-magnetic support member is formed into a laminate (a so-called composite film) composed of a plurality of layers and the roughness of a surface (the reverse side) of the non-magnetic support member opposite to a surface of the same, on which the ferromagnetic thin metal film will be formed, is made to be coarse.
Recently, elongation of permissible recording time, denser recording and larger quantity of information have been required. To elongate the recording time and enlarge the quantity of information, the overall thickness of the magnetic recording medium (in particular, a magnetic tape) must be reduced to increase the number of winding times around a reel so as to elongate the overall length of the magnetic recording medium which is accommodated in a cassette. It leads to a fact that a non-magnetic support member having a considerably reduced thickness as compared with the conventional non-magnetic support member has been employed.
However, a non-magnetic support member, such as the above-mentioned composite film, usually has a structure that the surface roughness of the surface, on which the magnetic layer will be formed, and that of the opposite surface (the reverse side) are different from each other. Therefore, when the magnetic recording medium is wound into a roll form, the reverse side of the non-magnetic support member and the right side (the surface) of the same are strongly pressed against each other. As a result, influences, such as undesirable printing and transcription of the reverse side, are exerted upon the surface, on which the magnetic layer will be formed, thus causing the surface roughness of the surface, on which the magnetic layer will be formed, to easily deteriorate. When a magnetic layer is formed on a non-magnetic support member of the foregoing type so that a tape medium is manufactured, the surface of the magnetic layer is directly affected by the influence of the roughness of the reverse side of the non-magnetic support member and thus the roughness of the reverse side is transferred. In this case designed surface roughness of the surface of the magnetic layer cannot be realized and the surface roughness sometimes deteriorates.
In recent years digitization of recording signals has been intended and research and development of digital signal processing apparatus have energetically been performed.
A recording/reproducing method (a digital method) using a digital signal process is able to compress and correct a signal, to be recorded, by using a mathematical method and thus the digital method has information processing performance decidedly superior to that of the conventional analog method. Therefore, the above-mentioned recording/reproducing method is expected to be further developed because it is capable of significantly improving the sound quality and image quality and enabling long recording time to be realized when sound or an image is recorded if the above-mentioned structure is adapted to a digital VTR apparatus.
Among digital type mediums, a magnetic recording medium, such as a DVC (a home digital video), must be adapted to high frequency recording (a shortened recording wavelength) to perform dense recording and accurate recording free from lacking of signals. Therefore, the relative velocity between the magnetic recording medium and the magnetic head has been required to be raised. In addition to raising the relative velocity, elongation of the recording time and enlargement of the quantity of information have been required. Under the above-mentioned circumstance, reduction in the magnetic tape to enable a longer magnetic tape to be accommodated and to elongate the magnetic tape has been required.
In particular, a magnetic tape of a type having the structure that the above-mentioned thin metal magnetic film is formed on a non-magnetic support member has superior coercive force and square ratio to those of a coating-type magnetic tape and the thickness of the magnetic layer can significantly be reduced. Thus, a variety of advantages can be realized in that demagnetization of recorded signals and thickness loss occurring during a reproducing operation can significantly be prevented, an excellent electromagnetic conversion characteristic at short wavelengths can be obtained and the packing density of a magnetic material can be raised. Therefore, the magnetic tape of the foregoing type is significantly be advantageous to serve as a medium, such as the DVC, for use in a digital process.
However, a very thin magnetic recording medium including a thin non-magnetic support member and, in particular, having a thickness of 8 .mu.m or smaller usually has difficulty in realizing a satisfactory handling easiness (workability) because adhesion of the non-magnetic support member takes place easily. Investigations of the DVC including a non-magnetic support member having a thickness of 8 .mu.m or smaller (having a thickness of 6 .mu.m) which is considerably smaller than that of the usual video tape (having a thickness of about 10 .mu.m or larger) have been made. As a result, excessive transcription to the right side takes place when the reverse side of the non-magnetic support member is made to be rough in order to improve the handling characteristic (the workability) after the magnetic tape has been wound into a roll. Thus, the electromagnetic conversion characteristic deteriorates excessively. Therefore, if the non-magnetic support member has a small thickness, improvement in the electromagnetic conversion characteristic cannot easily be made to be compatible with realization of the handling characteristic (the workability).
Accordingly, achievement of satisfactory workability in the processes for manufacturing the non-magnetic support member and the magnetic recording medium and prevention of deterioration in the surface characteristics of the magnetic layer occurring attributable to undesirable transcription of the reverse side and printing are required to make a satisfactory electromagnetic conversion characteristic to be compatible with the moving characteristic.